We injected the pericardial cavities of 25 sheep with either a killed bacterial or suspension or a control solution to produce pericarditis. The sheep have been studied at intervals of 3 hours to 1 year after injection. In vitro glutaraldehyde fixation of the epicardium and pericardium was performed. Epicardial and pericardial tissues were studied by light and electron microscopy. Killed bacteria produced acute and chronic inflammatory changes within the pericardial cavities. The epicardium demonstrated greater inflammation and necrosis than the pericardium, suggesting that a major component of acute and chronic "pericarditis" is related to alterations in the epicardium.